Control system for split axle drive mechanism

ABSTRACT

A part-time four-wheel drive vehicle has a transfer case and a split axle drive mechanism for the selectively driving two vehicle wheels. The slit axle drive mechanism has a clutch associated with one of the differential side gears for preventing back drive to the transfer case in the two-wheel drive mode. A control system for operating the clutch responsive to the operational mode of the transfer case comprises a vacuum motor and a solenoid operated two-way valve energized by way of a switch in the transfer case. The control system also includes a pneumatic time delay and a vacuum check valve.

This invention relates to a split axle drive mechanism and, moreparticularly, to a control system for operating the clutch of a splitaxle drive mechanism which is used in a part-time four-wheel drivevehicle.

A common drive configuration for a part-time four-wheel drive vehiclecomprises a transfer case having an input shaft driven by the vehicletransmission and two output shafts. One output shaft is drive connectedto the input shaft for continuously driving one set of vehicle wheels,usually the rear wheels, through a propeller shaft, differential andsplit axle. The second output shaft is connectable to the input shaft bya clutch or the like in the transfer case for selectively driving theother set of vehicle wheels, usually the front wheels, through a secondpropeller shaft, a differential and split axle. Two-wheel drive isprovided when the clutch in the transfer case is disengaged andfour-wheel drive when the clutch is engaged.

A long standing problem associated with part-time four-wheel driveconfigurations of the above-noted type is wear and power consumption inthe two-wheel drive mode. This is caused by the non-driven front wheelsback driving the drive line components between the non-driven wheels andthe clutch or comparable mechanism in the transfer case whichdisconnects the second or auxiliary output shaft from the transfer caseinput shaft.

A known solution to reducing wear and power consumption is the use of asingle clutch in the axle assembly for the selectively driven wheelswhich disconnects one of the selectively driven wheels from itsassociated side gear in the differential when the vehicle is in thetwo-wheel drive mode. See U.S. patent application Ser. No. 126,561, fora Split Axle Drive Mechanism filed by Laszlo Nagy on Mar. 3, 1980, nowU.S. Pat. No. 4,341,281.

The object of this invention is to provide a control system forautomatically operating the clutch in the split drive mechanismresponsive to the operational mode of the transfer case in a part-timefour-wheel drive vehicle.

Another object of this invention is to provide a control system whichuses components already on the vehicle as much as possible and therebyminimizing the number of additional components required by the system.

A feature of the invention is that the control system does not requireany power to disengage the clutch and, consequently, the control systemcontributes to fuel economy in the two-wheel drive mode.

Another feature of the invention is that the control system incorporatesa time delay between the shift to four-wheel drive in the transfer caseand the application of clutch engage forces in the split axle drivemechanism so that four-wheel drive can be selected while the vehicle isin motion.

Yet another feature of the invention is that the control system uses avacuum motor for engaging and disengaging the clutch and, consequently,the clutch fork can be operated directly by the vacuum motor without thenecessity of a complicated shifter mechanism as in the aforesaid patentapplication.

Still yet another feature of the invention is that the control systemincludes a vacuum check valve so that the system can operate off of theengine intake manifold.

Still yet another feature of the invention is that the control system iselectrically activated and deactivated by a pre-existing switchassociated with the transfer case.

Still yet another feature of the invention is that abusive clutchengagement, such as at full throttle, is not possible when the engine isthe vacuum source because the available vacuum is not sufficient toactuate the control system.

Still yet another feature of the invention is that the control systemuses the engine intake manifold as a vacuum source and a pre-existingswitch associated with the transfer case thereby minimizing the numberof additional components required by the system.

Other objects and features of the invention will become apparent tothose skilled in the art as the disclosure is made in the followingdetailed description of a preferred embodiment of the invention asillustrated in the accompanying sheets of drawing in which:

FIG. 1 is a schematic plan view of a part-time four-wheel drive vehiclehaving a split axle drive mechanism and a control system for operatingthe clutch thereof in accordance with my invention.

FIG. 2 is a partially sectioned plan view of the split axle drivemechanism shown in FIG. 1.

FIG. 3 is a schematic view showing details of the control system and itsrelationship to other vehicle components.

Referring now to the drawing and particularly to FIG. 1, there is showna schematic plan view of a part-time four-wheel drive vehicle,comprising an internal combustion engine 10, transmission 12 andtransfer case 14 mounted on a vehicle chassis (not shown). The engine 10and transmission 12 are well-known components as is the transfer case 14which typically has an input shaft (not shown), a main output shaft 16and an auxiliary output shaft 18. The main output shaft 16 is driveconnected to the input shaft in the transfer case 14 and is customarilyaligned with it. The auxiliary output shaft 18 is drive connectable tothe input shaft by a clutch or the like in the transfer case 14 andcustomarily offset from it. The transfer case clutch is actuated by asuitable selector mechanism (not shown) which is generally remotelycontrolled by the vehicle driver.

The main output shaft 16 is drivingly connected to a rear propellershaft 20 which in turn is drivingly connected to a rear differential 22.The rear differential 22 drives the rear wheels 24 through split axleparts in a well-known manner.

The auxiliary output shaft 18 is drivingly connected to a frontpropeller shaft 26 which in turn is drivingly connected to a split axledrive mechanism 28 for selectively driving the front wheels 30 throughsplit axle parts.

The Split Axle Drive Mechanism

As shown in FIGS. 2 and 3, the split axle drive mechanism 28 includes anautomotive type differential 32 inside a housing 34. The differential 32has a drive shaft 36 and a differential case 38 rotatably mounted in thehousing 34 on orthogonally relaxed axes. The drive shaft 36 is thedifferential input and has an external yoke 40 at one end foruniversally coupling the drive shaft 36 to the front propeller shaft 26.The internal end of the drive shaft 36 has an integral driving pinion 42which meshes with a ring gear 44 attached to the differential case 38.The differential case 38 carries a plurality of rotatable pinion gears46 mounted on a cross pin 48. The pinion gears 46 mesh with side gears50 and 52 which are splined to the end of the stub shafts 54 and 56respectively. The stub shafts 54 and 56 are rotatably mounted in thehousing 34 on the differential case axis. These stub shafts arerotatable relative to each other and to the differential case. Thedifferential 32 as thus far described and its mode of operation arewell-known.

The split drive axle mechanism 28 further includes a positive clutch 58which changes the mode of operation of the differential 32 and makes itparticularly useful for the selectively driven wheels in a part-timefour-wheel drive vehicle. As shown in FIG. 3, the clutch 58 comprises anintegral spline wheel 60 at the outer end of the stub shaft 54 and amatching spline wheel 62 attached to the inner end of an extension shaft64. The extension shaft 64 has its inner end journalled in the hollowouter end of the stub shaft 54 and its outer end journalled in a bearing(not shown) at the remote end of an extension tube 66 attached to thehousing 34.

The clutch 58 further includes an internally splined sleeve 68 which isslidably mounted on the spline wheel 60. The splined sleeve 68 isshiftable between a disengaged position (shown in solid lines in FIGS. 2and 3) and an engaged position (shown in phantom lines in FIG. 3) whereit couples the spline wheels 60 and 62.

The split axle drive mechanism 42 is attached to the vehicle chassis bymeans of a housing bracket (not shown) and a bracket 70 on the extensiontube 66.

The split axle drive mechanism 28 has two outputs for the respectivesplit axle parts associated with the respective front wheels 30. Oneoutput is the stub shaft 54, clutch 58 and extension shaft 64 which hasan external flange 72 for attaching one of the split axle parts. Theother output is the stub shaft 56 which has an external flange 74 forattaching the other split axle part.

Suitable split axle parts, commonly referred to as half shafts, arewell-known from front wheel drive automobiles. These may be used forconnecting the split axle drive mechanism 28 to the front wheels 30. Thedrawings schematically illustrate a common type of half shaft fordriving connection to independently suspended steerable vehicle wheelscomprising an axle shaft 76 having a plunging universal joint 78 at itsinboard end adapted for connection to an output such as the flange 72 or74 and the well-known Rzeppa-type universal joint 80 at its outboard endadapted to be connected to the vehicle wheel 30.

The split axle drive mechanism 28 also includes a shifter 81 foroperating the clutch 58. The shifter 81 as shown in FIG. 3 comprises afork 82 having its tines engaged in an external groove of the sleeve 68and its base slidably mounted on a slide 84. The fork 82 is positionedon the slide 84 by opposed coil springs 86 and 88. The slide 84 itselfis translated by a push-pull cable 90. FIG. 3 shows the fork 82 and theslide 84 in the clutch disengaged position in solid lines. The clutch 58is engaged by moving the slide 84 to the left from the solid lineposition shown in FIG. 3. This loads the spring 88 which in turn biasesthe fork 82 and sleeve 68 toward the left. The sleeve 68 then slidesinto engagement with the spline wheel 62 under the action of spring 88when their respective splines align in a complementary manner. Theclutch 58 is disengaged by returning the slide 84 to the position shownin FIG. 3. This loads the spring 86 which in turn returns the slide 84and fork 82 to the clutch disengaged position when the biasing force ofspring 86 is sufficient to overcome the torque load on the engagedsplines of spline wheel 62 and sleeve 68.

The Control System

A control system for operating the clutch 58 via the push-pull cable 90and shifter 81 is shown in the lower portion of FIG. 3.

The control system comprises a vacuum motor 92, a solenoid operatedtwo-way slide valve 94, a vacuum check valve 96, three rubber conduitsor hoses 98, 100 and 102 and an orifice device 103.

The vacuum motor 92 comprises a hard plastic cup shaped shell 104 and aflexible cup shaped diaphragm 106 attached together rim-to-rim to form acollapsible chamber 108. The bottom wall of the diaphragm 106 isreinforced by plates 110 and 112 which are on opposite sides of thebottom wall and riveted together. The diaphragm 106 is normally extendedas shown in FIG. 3 and biased into the extended position by a coilspring 114 inside the chamber 108. The hard plastic shell 104 has anipple 116 which forms a port 117 for evacuating or venting the chamber108.

The push-pull cable 90 which operates the shifter 81 for the clutch 58is attached to an eyelet of the outer plate 112 as shown in FIG. 3 sothat the clutch 58 is disengaged when the chamber 108 is vented toatmosphere and the diaphragm 106 of the vacuum motor 92 is extended.

The vacuum motor 92 is mounted on a U-shaped bracket 118 which in turnis fixedly mounted in the engine compartment of the vehicle, such as byfastening the bracket 118 to a body panel as schematically representedin FIG. 1.

The two-way slide valve 94 can also be conveniently mounted on thebracket 118 as shown in FIG. 1. However, the slide valve 94 is shown ina detached position in FIG. 3 for clarity.

The two-way slide valve 94 comprises a sheet metal cup 119 with aplastic spool secured in it to provide a cylindrical valve chamber 120.The valve chamber 120 has coaxial ports 122,124 at opposite ends and aradial port 126. Ports 122 and 124 are vent and vacuum ports,respectively. The orifice device 103 is a bias cup having a small holethrough its bottom wall. The cup is mounted in the outer end of theradial port 126 and the radial port 126 is connected to the vacuum motor92 by the rubber hose 98.

A slide member 128 is disposed in the valve chamber 120. The slidemember 128 has stems 130,132 at its opposite ends which cooperate withthe respective vent and vacuum ports 122 and 124. The slide member 128is biased by a coil spring 134 to an extended position where the stem132 closes the vacuum port 124 as shown in FIG. 3. Consequently, thevacuum motor 92 is normally vented via the open vent port 122.

The valve chamber 120 is surrounded by a solenoid coil 136 which, whenenergized, retracts the slide member 128 so that the stem 130 closes thevent port 122 and the vacuum port 124 is opened. The enlarged centersection 129 of the slide member 128 is a cylinder with four equallyspaced flats 131. The cylinder pilots the slide member 128 in the valvechamber 120 while the flats 131 permit flow from one end of the valvechamber 120 to the other, particularly in the vent mode illustrated inFIG. 3 where air flows from the vent port 122 to the vacuum motor 92 viathe radial port 126.

The vacuum port 124 is connected to the vacuum check valve 96 by therubber hose 100. The vacuum check valve 96 comprises a plastic housing137 having a nipple at each end which form respective suction anddischarge ports 138,139 for a valve chamber 140. The suction port 138 isconnected to the vacuum port 124 of the slide valve 94 by the hose 100as indicated above. The discharge port 139 is connected by hose 102 to avacuum source, such as the intake manifold of the internal combustionengine 10.

The valve chamber 137 has an internal apertured wall 141 which supportsa coil spring 143 and a flat plug 142 which is biased by the coil springto block the suction port 138. The vacuum check valve 96 permits airflow from the slide valve 94 to the vacuum source that is, in thedirection of the arrow 144 shown in FIG. 3 but prevents air flow fromthe vacuum source to the slide valve 94. The vacuum check valve 96prevents the engine from bleeding down the vacuum motor 92 when thechamber 108 is evacuated.

The solenoid operated two-way slide valve 94 has a plastic socket 146 atthe end which has the vent port 122. The socket 146 houses a pair ofmale blade terminals 148,150 attached to the respective ends of thesolenoid coil 136. The socket 146 also houses a filter 152 for the ventport 122.

The electric circuit for energizing the solenoid coil 136 is shownschematically in FIG. 3. By way of background, transfer cases forpart-time four-wheel drive vehicles commonly include an electric switchwhich is closed when the transfer case is in the four-wheel drive mode.The closed switch completes a circuit to an indicator light on thevehicle instrument panel to advise the vehicle driver that the vehicleis in the four-wheel drive mode. See for instance, U.S. Pat. No.3,283,298 issued to Edgar F. Kaiser on Nov. 1, 1966.

FIG. 3 schematically illustrates a transfer case 14 having a switch 154,it being understood that the switch 154 is operatively connected withthe transfer case 14 so that the switch 154 opens when the transfer case14 is in the two-wheel drive mode and closes when the transfer case 14is in the four-wheel drive mode.

The switch 154 is electrically connected in series with the vehiclebattery 156 and two branch circuits--one having an indicator light 158and the other having the solenoid coil 136. Consequently, the solenoidcoil 136 is also energized when the indicator light 158 is litresponsive to the transfer case 14 being in the four-wheel drive mode.

Operation

The two-wheel drive mode is illustrated in FIGS. 2 and 3. In this mode,the drive to the auxiliary output shaft 18 is disconnected in thetransfer case and, consequently, the switch 154 is open. The slidemember 128 is extended under the action of spring 134, in a positionblocking the vacuum port 124, and opening the vent port 122. The clutch58 is disengaged and held in the disengaged position by the coil spring114 in the vacuum motor 92 acting on the shifter 81 via the push-pullcable 90.

When the vehicle is driven in the two-wheel drive mode, the lower wheel30 shown in FIG. 1 back drives the differential side gear 52 but theupper wheel 30 does not back drive the differential side gear 50 becausethe clutch 58 is disengaged. Since the side gear 50 does not have anyload, the side gear 52 merely counter-rotates the side gear 50 throughthe pinion gears 46. Hence, there is no back drive to the differentialcase 38, drive shaft 36 (differential input), front propeller shaft 36,auxiliary output shaft 18 and other transfer case components connectedto the auxiliary output shaft 18 ahead of the disconnect in the transfercase 14. This mode of operation eliminates the major portion of wear andpower consumption which would result from back drive of both wheels 30.

It should also be noted that the control system does not require anypower for operation in the two-wheel drive mode since the vacuum motor92 is vented and the solenoid operated two-way slide valve 94 isdeenergized. Consequently, the control system itself enhances fueleconomy in the two-wheel drive mode.

When the four-wheel drive mode is selected by the vehicle operator, theauxiliary output shaft 18 is drive connected to the input shaft in thetransfer case 14 and the switch 154 is closed, setting off two chains ofevents which result in the clutch 58 automatically being engaged.

The transfer case output shaft 18 now drives the drive shaft 36(differential input) and the differential case 38. The driven orrotating differential case 38 in turn reverses the counter-rotating sidegear 50 so that the side gear 50 and stub shaft 54 rotate in the samedirection as the side gear 52 and the extension shaft 64 which aredriven by the respective front vehicle wheels 30. In time, the drivendifferential case 38 tends to synchronize the speeds of the stub shaft54 and the extension shaft 64.

In the meantime, the closed switch 154 energizes the solenoid coil 136of the two-way slide valve 94 retracting the slide member 128. Theretracted slide member 128 closes the vent port 122 and opens the vacuumport 124 which connects the chamber 108 of vacuum motor 92 to the vacuumsource provided by the internal combustion engine 10 through hose 102,vacuum check valve 96, hose 100, vacuum port 124, radial port 126,orifice device 103, rubber hose 98 and port 117.

The vacuum chamber 108 is then evacuated, producing a clutch engageforce which collapses the diaphragm 106 against the action of spring 114and pulls the slide 84 to the left from the solid line position shown inFIG. 3 via the cable 90. Movement of the slide 84 loads the coil spring88 which biases the shifter fork 82 and sleeve 68 toward the clutchengage position. The sleeve 68 then slides into engagement with thespline wheel 62 under the action of spring 88 when their respectivesplines align in a complementary manner. When the clutch 58 is engaged,as shown in phantom in FIG. 3, both front wheels 30 are driven and thesplit axle drive mechanism 28 acts as a conventional differential.

The orifice device 103 provides a time delay of approximately two tothree seconds between the time that the switch 154 is closed and thevacuum chamber 108 is evacuated. This time delay is provided so thatrotation of the counter-rotating side gear 50 is reversed and the sleeve68 is rotating in the same direction as the spline wheel 62 before anysubstantial clutch engage force is produced by the vacuum motor 92. Thistime delay feature permits the vehicle operator to shift the transfercase 14 from two-wheel drive to four-wheel drive while the vehicle is inmotion.

Another feature which should be noted is the vacuum check valve 96 whichkeeps the chamber 108 evacuated once it is evacuated by the engineintake manifold. This feature prevents the evacuated chamber 108 frombeing bled down by a low vacuum in the engine intake manifold such as ina steep, four-wheel drive, hillclimb.

The clutch 58 is also automatically disengaged when the vehicle isreturned to the two-wheel drive. When the two-wheel drive mode isselected at the transfer case 14, the switch 154 opens deenergizing thesolenoid coil 136. The slide member 128 is then biased by spring 134 tothe position shown in FIG. 3 where the vacuum port 124 is closed and thevent port 122 is opened. The chamber 108 is now vented and the diaphragm106 extends, aided by the coil spring 114. This in turn pushes the slide84 via the push-pull cable 90 to the clutch disengaged position shown inFIG. 3 which loads the coil spring 86. The coil spring 86 then biasesthe fork 82 and sleeve 68 toward the solid line position shown in FIG.3. The clutch 58 then automatically disengages when the force of spring86 is sufficient to overcome the torque loading between the spline wheel62 and the sleeve 68 which usually occurs with a slight deceleration ofthe vehicle. When the clutch 58 is disengaged, there is no back drive tothe differential case 38 as indicated earlier.

It should be noted that shifter 81 provides a redundancy when used inconjunction with the control system illustrated in FIG. 3. The coilsprings 86 and 88 bias the sleeve 68 toward the disengaged and engagedposition, respectively, so that the clutch 58 is not engaged anddisengaged with excessive force which could happen if the shift fork 82was operated by a solid mechanical linkage.

Since the vacuum motor 92 itself provides controlled clutch engagementand disengagement forces, it is possible to simplify the shifter 81 byeliminating the springs 86 and 88 and fixing the shift fork 82 on theslide 84 so that the push-pull cable 90 moves the shift fork 82directly.

It wish it to be understood that I do not desire to be limited to theexact details of construction shown and described, for obviousmodifications will occur to a person skilled in the art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A control system forautomatically operating the clutch in a single clutch split axle drivemechanism responsive to an operational mode of a transfer case, withwhich the split axle drive mechanism is used in a part-time four-wheeldrive vehicle powered by an internal combustion engine, comprising:avacuum motor mechanically connected to the clutch so that the clutch isbiased toward disengagement when the vacuum motor is vented and biasedtoward engagement when the vacuum motor is evacuated, conduit meansconnecting the vacuum motor to a vacuum source provided by the internalcombustion engine, said conduit means including a two-way valve whichhas a first operative position when the vacuum motor is connected to avent and a second operative position where the vacuum motor is connectedto the vacuum source for evacuating the vacuum motor, means operativelyconnected to the transfer case positioning the two-way valve in thesecond operative position when the transfer case is in a four-wheeldrive mode whereby the clutch is automatically engaged when four-wheeldrive is selected, and said two-way valve being mechanically positionedin the fist operative position when the transfer case is in thetwo-wheel drive mode whereby the control system does not require anypower for operation in the two-wheel drive mode.
 2. A control system forautomatically operating the clutch in a single clutch split axle drivemechanism responsive to an operational mode of a transfer case, withwhich the split axle drive mechanism is used in a part-time four-wheeldrive vehicle powered by an internal combustion engine, comprising:avacuum motor operatively connected to the clutch so that the clutch isbiased toward disengagement when the vacuum motor is vented and biasedtoward engagement when the vacuum motor is evacuated, conduit meansconnecting the vacuum motor to a vacuum source provided by the internalcombustion engine, said conduit means including a two-way valve whichhas a first operative position where the vacuum motor is vented and asecond operative position where the vacuum motor is evacuated by thevacuum source, means positioning the two-way valve in the secondposition when the transfer case is in a four-wheel drive mode wherebythe clutch is automatically engaged when four-wheel drive is selected,and time delay means in the conduit means for delaying clutch engagementto a time after the transfer case is shifted to four-wheel drive, sothat the clutch parts are rotating in the same direction and four-wheeldrive can be selected while the vehicle is in motion.
 3. A controlsystem for automatically operating the clutch in a single clutch splitaxle drive mechanism responsive to an operational mode of a transfercase, with which the split axle drive mechanism is used in a part-timefour-wheel drive vehicle powered by an internal combustion engine,comprising:a vacuum motor mechanically connected to the clutch so thatthe clutch is biased toward disengagement when the vacuum motor isvented and biased toward engagement when the vacuum motor is evacuated,conduit means connecting the vacuum motor to the intake manifold of theinternal combustion engine for evacuating the vacuum motor, said conduitmeans including a two-way valve which has a first operative positionwhere the vacuum motor is vented and a second operative position wherethe vacuum motor is connected to the vacuum source, electric meansoperatively connected to the transfer case positioning the two-way valvein the second operative position when the transfer case is in afour-wheel drive mode whereby the clutch is automatically engaged whenfour-wheel drive is selected, and a vacuum check valve in the conduitmeans between the two-way valve and the intake manifold for preventingbleed down of the evacuated vacuum motor by the intake manifold.
 4. Acontrol system for automatically operating the clutch in a single clutchsplit axle drive mechanism responsive to an operational mode of atransfer case, with which the split axle drive mechanism is used in apart-time four-wheel drive vehicle powered by an internal combustionengine, comprising:a vacuum motor mechanically connected to the clutchso that the clutch is biased toward disengagement when the vacuum motoris vented and biased toward engagement when the vacuum motor isevacuated, conduit means connecting the vacuum motor to the intakemanifold of the internal combustion engine for evacuating the vacuummotor, said conduit means including a two-way valve which has a firstoperative position where the vacuum motor is vented and a secondoperative position where the vacuum motor is connected to the vacuumsource, spring means positioning the valve in the first operativeposition, electric means positioning the two-way valve in the secondoperative position when the transfer case is in a four-wheel .Iadd.drive.Iaddend.mode whereby the clutch is automatically engaged whenfour-wheel drive is selected, pneumatic time delay means in the conduitmeans between the vacuum motor and the two-way valve for delaying theclutch engagement for a predetermined mininum amount of time after thetransfer case is shifted to four-wheel drive so that the clutch partsare rotating in the same direction when four-wheel drive is selectedwhile the vehicle is in motion, and a vacuum check valve in the.[.condition.]. .Iadd.conduit .Iaddend.means between the two-way valveand the intake manifold for preventing bleed down of the evacuatedvacuum motor by the intake manifold.
 5. In a part-time four-wheel drivevehicle having a source of vacuum, a transfer case having a two-wheeldrive operating mode and a four wheel drive operating mode, and a splitaxle drive mechanism having a single clutch moveable between disengagedand engaged positions for the two-wheel drive and four-wheel driveoperating modes, respectively,a control system for automaticallyoperating the clutch responsive to the operational mode of the transfercase, comprising: a vacuum motor, means operatively connecting thevacuum motor to the clutch for biasing the clutch toward the engagedposition when the vacuum motor is evacuated, and biasing the clutchtoward the disengaged position when the vacuum motor is vented toatmosphere, conduit means connected between the vacuum motor and thevacuum source, said conduit means including valve means moveable betweenfirst and second positions for alternately connecting the conduit meansto atmosphere or to the vacuum source, means normally locating the valvemeans in the first position to connect the conduit means to atmosphereand maintain the clutch disengaged, signal means actuated by thetransfer case when in the four-wheel drive operating mode for indicatingsuch mode to the vehicle operator, and means actuated by the transfercase concurrently with actuation of the signal means for moving thevalve means to the second position to connect the vacuum motor to thevacuum source and evacuate the vacuum motor to bias the clutch towardthe engaged position when the transfer case is in the four-wheel driveoperating mode, and time delay means delaying evacuation of the vacuummotor by the vacuum source to a time after the transfer case is in thefour-wheel drive operating mode so that the clutch parts are rotating inthe same direction and four-wheel drive can be selected while thevehicle is in motion.
 6. A control system for automatically operatingthe clutch in a single clutch split axle drive mechanism responsive toan operational mode of a transfer case, with which the split axle drivemechanism is used in a part-time four-wheel drive vehicle powered by aninternal combustion engine, comprising:a vacuum motor operativelyconnected to the clutch so that the clutch is biased towarddisengagement when the vacuum motor is vented and biased towardengagement when the vacuum motor is evacuated, conduit means connectingthe vacuum motor to a vacuum source provided by the internal combustionengine, said conduit means including a two-way valve which has a firstoperative position where the vacuum motor is vented and a secondoperative position where the vacuum motor is evacuated by the vacuumsource, means positioning the two-way valve in the second position whenthe transfer case is in a four-wheel drive mode whereby the clutch isautomatically engaged when four-wheel drive is selected, and time delaymeans for delaying clutch engagement to a time after the transfer caseis shifted to four-wheel drive, so that the clutch parts are rotating inthe same direction and four-wheel drive can be selected while thevehicle is in motion.
 7. A control system for automatically operatingthe clutch in a single clutch split axle drive mechanism responsive toan operational mode of a transfer case, with which the split axle drivemechanism is used in a part-time four-wheel drive vehicle powered by aninternal combustion engine, comprising:a vacuum motor mechanicallyconnected to the clutch so that .[.that.]. clutch is biased towarddisengagement when the vacuum motor is vented and .[.baised.]..Iadd.biased .Iaddend.toward engagement when the vacuum motor isevacuated, conduit means connecting the vacuum motor to the intakemanifold of the internal combustion engine for evacuating the vacuummotor, said conduit means including a two-way valve which has a firstoperative position where the vacuum motor is vented and a secondoperative position where the vacuum motor is connected to the vacuumsource, spring means positioning the valve in the first operativeposition, means positioning the two-way valve in the second operativeposition when the transfer case is in a four-wheel drive mode wherebythe clutch is automatically engaged when four-wheel drive is selected,pneumatic time delay means for delaying the clutch engagement for apredetermined minimum amount of time after the transfer case is shiftedto four-wheel drive so that the clutch parts are rotating in the samedirection when four-wheel drive is selected while the vehicle is inmotion, and a vacuum check valve in the conduit means between thetwo-way valve and the intake manifold for preventing bleed down of theevacuated vacuum motor by the intake manifold.
 8. In a part-timefour-wheel drive vehicle having a source of vacuum, a transfer casehaving a two-wheel drive operating mode and a four-wheel drive operatingmode, and a split axle drive mechanism having a single clutch moveablebetween disengaged and engaged positions for the two-wheel drive andfour-wheel drive operating modes, respectively,a control system forautomatically operating the clutch responsive to the operational mode ofthe transfer case, comprising: a vacuum motor; means operativelyconnecting the vacuum motor to the clutch for biasing the clutch towardthe engaged position when the vacuum motor is evacuated, and biasing theclutch toward the disengaged position when the vacuum motor is vented toatmosphere, conduit means connected between the vacuum motor and thevacuum source, said conduit means including valve means moveable betweenfirst and second positions for alternately connecting the conduit meansto atmosphere or to the vacuum source, means normally locating the valvemeans in the first position to connect the conduit means to atmosphereand maintain the clutch disengaged, means actuated by the transfer casefor moving the valve means to the second position to connect the vacuummotor to the vacuum source and evacuate the vacuum motor to bias theclutch toward the engaged position when the transfer case is in thefour-wheel drive operating mode, and time delay means delayingevacuation of the vacuum motor by the vacuum source to a time after thetransfer case is in the four-wheel drive operating mode so that theclutch parts are rotating in the same direction and four-wheel drive canbe selected while the vehicle is in motion. .Iadd.
 9. A control systemfor automatically operating the clutch in a single clutch split axledrive mechanism responsive to an operational mode of a transfer case,with which the split axle drive mechanism is used in a part-timefour-wheel drive vehicle powered by an internal combustion enginecomprising:a motor operatively connected to the clutch so that theclutch is biased toward disengagement when a first signal iscommunicated to the motor and biased toward engagement when a secondsignal is communicated to the motor, control means having a firstoperative position where the first signal is communicated to the motorand a second operative position where the second signal is communicatedto the motor, means positioning the control means in the secondoperative position when the transfer case is in a four-wheel drive modewhereby the clutch is automatically engaged when four-wheel drive isselected, and time delay means for delaying clutch engagement to a timeafter the transfer case is shifted to four-wheel drive, so that theclutch parts are rotating in the same direction and four-wheel drive canbe selected while the vehicle is in motion. .Iaddend. .Iadd.
 10. Acontrol system for automatically operating the clutch in a single clutchsplit axle drive mechanism responsive to an operational mode of atransfer case, with which the split axle drive mechanism is used in apart-time four-wheel drive vehicle powered by an internal combustionengine comprising:a fluid motor operatively connected to the clutch sothat the clutch is biased toward disengagement when a first pressurecondition is communicated to the fluid motor and biased towardengagement when a second pressure condition is communicated to the fluidmotor, conduit means connecting the fluid motor to a fluid pressuresource, said conduit means including a valve which has a first operativeposition where the first pressure condition is communicated to the fluidmotor and a second operative position where the second pressurecondition is communicated to the fluid motor, means positioning thevalve in the second operative position when the transfer case is in afour-wheel drive mode whereby the clutch is automatically engaged whenfour-wheel drive is selected, and time delay means for delaying clutchengagement to a time after the transfer case is shifted to four-wheeldrive, so that the clutch parts are rotating in the same direction andfour-wheel drive can be selected while the vehicle is in motion..Iaddend. .Iadd.
 11. A control system enabling selection of four-wheeldrive during two-wheel driven motion of a vehicle having a transfer casewhich includes a clutch or the like actuable to drivingly connect enginetorque with a propeller shaft connected with the differential input of adifferentially split axle drive mechanism having a first differentialoutput drive coupled with a first wheel and a second differential outputdisconnectably coupled to a second wheel by an axle clutch enabling thetwo-wheel drive mode to include disconnection of the axle clutch as wellas deactuation of the transfer case clutch so that the propeller shaftand differential input remain at fuel and wear saving rest while theroad driven first wheel back drives the second differential output incounterrotation relative to the second wheel, said control systemcomprising:motor means connected to the axle clutch to actuate the axleclutch to connectably couple the second differential output and thesecond wheel, and control means operably associated with the transfercase and adapted to operate the motor means and effect axle clutchengagement, said control means having associated time delay means sothat axle clutch engagement occurs subsequent to actuation of thetransfer case clutch or the like to provide engine driven rotation ofthe then resting propeller shaft and the differential input and providereversal of the counterrotation of the second differential output intosame direction rotation with the second wheel at a substantiallysynchronized rate therewith whereby four-wheel drive can be selectedwhile the vehicle is in two-wheel drive motion. .Iaddend. .Iadd.
 12. Acontrol system enabling selection of four-wheel drive during two-wheeldriven motion of a vehicle having a transfer case which includes aclutch or the like actuable to drivingly connect engine torque with apropeller shaft connected with the differential input of adifferentially split axle drive mechanism having a first differentialoutput drive coupled with one wheel and a second differential outputdisconnectably coupled to a second wheel by an axle clutch enabling thetwo-wheel drive mode to include disconnection of the axle clutch as wellas deactuation of the transfer case clutch so that the propeller shaftand differential input remain at fuel and wear saving rest while theroad driven first wheel back drives the second differential output incounterrotation relative to the second wheel, said control systemcomprising:motor means operatively connected to the axle clutch forengaging the axle clutch; motor control means adapted to operate themotor means to engage the axle clutch in response to four-wheel driveselection and attendant actuation of the transfer case clutch to therebyconnectably couple the second differential output with the second wheel;and means for delaying the axle clutch engagement for an amount of timesubsequent to transfer case clutch actuation to enable initiation ofengine driven rotation of the then resting propeller shaft anddifferential input and concomitant reversal of the counterrotation ofthe second differential output into rotation in the same direction withthe second wheel and at a substantially synchronized rate therewithwhereby the transfer case clutch and axle clutch can be selected totheir respective four-wheel drive operating modes while the vehicle isin motion. .Iaddend. .Iadd.13. A control system enabling selection offour-wheel drive during two-wheel driven motion of a vehicle having atransfer case which includes a clutch or the like actuable to drivinglyconnect engine torque with a propeller shaft connected with thedifferential input of a differentially split axle drive mechanism havinga first differential output coupled with one wheel and a seconddifferential output disconnectably coupled to a second wheel by an axleclutch enabling the two-wheel drive mode to include disconnection of theaxle clutch as well as deactuation of the transfer case clutch so thatthe propeller shaft and differential input remain at fuel and wearsaving rest while the road driven first wheel back drives the seconddifferential output in counterrotation relative to the second wheel,said control system comprising: a fluid motor operatively connected tothe axle clutch so that the axle clutch is disengaged in response to afirst fluid pressure condition and engaged in response to a second fluidpressure condition; valve means having a first operative position tocommunicate the first fluid pressure condition to the fluid motor and asecond operative position for communicating the second fluid pressurecondition to the fluid motor; means positioning the valve means in thesecond operative position when the transfer case clutch is engagedwhereby the axle clutch is automatically engaged in response toengagement of the transfer case clutch and attendant engine drivenrotation of the propeller shaft and differential input; and means fordelaying axle clutch engagement to a time after the transfer case clutchis engaged to assure reversal of the counterrotation of the seconddifferential output and rotation of the second differential output at asubstantially synchronized rate with the second wheel whereby the axleclutch can be engaged while the vehicle is in motion. .Iaddend.